Studies On Predictive Biomarkers Of PARP Inhibitors And On Characteristics And Machanisms Of Tumor Resistance To Them

BRCA1/2 is the most widely-used biomarker of poly(ADP-ribose)polymerase(PARP)inhibitors in the clinic.However,the clinical rate of cancer patients with BRCA mutations is only 30%~50%,meaning more than half of patients with BRCA1-deficient cancers are faced with futile treatments with PARP inhibitors.Here we report the combination of 53BP1 with BRCA1 as a biomarker of PARP inhibitor sensitivity.Based on mRNA levels of four homologous recombination(HR)repair genes and PARP inhibitor sensitivity,we selected BRCA1-deficient MDA-MB-436 cells to conduct RNA interference.Only reduced 53BP1 expression was found to lower the simmiparib sensitivity.We then generated 53BP1-/-/BRCA1-/-variants and found that depleting 53BP1 impaired PARP inhibitor sensitivity with 36.7-fold increases in IC50 s.Consistently,53BP1 loss alleviated cell cycle arrest and apoptosis and partially restored HR function.Importantly,53BP1 depletion caused a dramatic fall of growth inhibition by PARP inhibitors in vivo.Re-expressing 53BP1 in the dual-deficient cells restored PARP inhibitor sensitivity and the levels of HR regulators.In considering that above 10% BRCA1-deficient breast and ovarian cancers negatively express 53BP1,our results indicate that introducing the combined biomarker of 53BP1 with BRCA1 into patient selection is likely to reduce the futile treatments with PARP inhibitors.BRCA1/2-deficient cells have compromised DNA repair and are sensitive to PARP inhibitors.Despite initial responses,BRCA1/2-defective tumors can exhibit resistance to PARP inhibitors via multiple mechanisms and the development of resistance limits clinical efficacy.To study PARP inhibitor resistance,we cultured the triple negative breast cancer cell line MDA-MB-436 in the presence of four kinds of the PARP inhibitors.The acquired drug resistant clones are highly resistant to other PARP inhibitors and cross-resistant to DNA cross linker and topoisomerase Ι inhibitors.Moreover,PARP inhibitors scarcely affect cell cycle and apoptosis of resistant cells of MDA-MB-436.Here,we show the BRCA1 protein detected in four kinds of resistant clones with an N-terminal BRCA1 antibody is C-terminal truncated and consequently not recognized by a C-terminal antibody.Therefore,it is likely that restoration of HR rapair function by secondary mutation of BRCA1 in MDA-MB-436 cells accounts for the resistance to PARP inhibitors.